1. Field of the Invention
This invention relates generally to remote control devices and, more particularly, to devices utilizing dual tone multi-frequency (DTMF) systems for remotely activating control devices via a single channel communications link.
2. Prior Art
There are many industrial applications for remote control systems. Many such systems require a high degree of reliability. A stray signal in such a system could cause erratic behavior with dangerously heavy equipment. In systems where a high degree of reliability is essential, remote control has been provided by an array of momentary action switches, each of which is connected to a corresponding control relay by an individual wire, commonly referred to as a multi-channel (or multi-wire) link. An example of such a control system can be found in multifunction transferring devices (e.g. overhead cranes, hydraulic lifts, etc.). In such devices, individual relays are provided for activating each function possible (e.g. "off", "on", "up", "down", "left", "right", "forward", or "reverse"). A multi-wire connection would normally allow any valid combination of control functions, e.g. "up-right-forward", to be activated simultaneously. Also, a multi-wire connection allows for a smooth transition between combinations of control functions, e.g. from "up-right" to "up-right-forward", eliminating discontinuous action when switching between such overlapping control functions. Thus, an operator could alternately switch between commands with no break in the common (in the above case, the "upright") action of the controlled device. An inherent problem of multi-wire connections is the lack of mobility of the operator. Frequently, due to the physical limitations of multi-wire controllers, operators are required to position themselves dangerously near, or even under, the load being handled. However, multi-wire devices have been preferred due to the direct connection between the remote control and the control device, thereby reducing the likelihood of a stray signal or other outside interference influencing the control device.
The inherent noise immunity and reliability of DTMF encoding/decoding systems make them especially acceptable for applications in controlling multifunction systems over a single channel communications link, especially where interference-free operation is essential. As opposed to a multi-wire link, a single channel link is not dependent upon the DTMF encoding/decoding units and may thus be chosen to fit the user's needs. The simple single channel link may be a single wire pair or any elementary single channel radio, optical, or sonic device. Also, the recent advent of specialized DTMF encoding and decoding integrated circuits allows DTMF based remote control systems to be low in cost as well as simple, efficient, small in size and lightweight. However, conventional DTMF encoding/decoding techniques allow only a single encoding switch (of which 16 are possible in a standard system) to be activated at any one time, producing and ultimately receiving a dual tone representing a position (row and column) in a four-row, four-column matrix. Therefore, the simultaneous multi-function control capability of a multi-wire link has not been heretofore simulated by conventional DTMF encoding/decoding techniques.